Catalytic reactors for carrying out endothermic or exothermic reactions are well known in the art particular examples being reactors for the endothermic steam reforming of hydrocarbons and reactors for the exothermic methanol synthesis reaction. The reactions are typically carried out in tubes loaded with a suitable solid catalyst through which a process gas stream comprising the reactants is passed at elevated pressure. A plurality of tubes is arranged vertically or horizontally in the reactor. The tubes run in parallel along the major axis of the catalytic reactor, while a heat-exchanging medium outside heats or cools the tubes. The solid catalyst inside the tubes provides a catalyst bed in which the required chemical reactions take place. The catalyst can be provided as solid particles or as a coated structure, for example as a thin layer fixed on the inner wall of the tubes in steam reforming reactors.
In another reactor configuration comprising a plurality of tubes the solid catalyst particles may be disposed outside said tubes hereinafter referred to also as heat transfer tubes, whilst the heat exchanging medium passes inside. The solid catalyst outside the heat transfer tubes provides the catalyst bed in which the required chemical reactions take place.
A particular type of heat transfer tube used in heat exchange reactors is the so-called double-tube. A double-tube is basically an arrangement of two substantially concentric tubes. The space in between the tube walls defines an annular cavity through which a heat-exchanging medium can flow so that temperature control is achieved by indirect heat exchanging between a process stream passing through the catalyst bed and said heat-exchanging medium. In an arrangement comprising a plurality of double-tubes, the solid catalyst in the bed is advantageously disposed both outside and inside the double-tubes.
Yet another type of heat transfer tubes in heat exchange reactors is an arrangement of two substantially concentric tubes, but where the inner tube is closed at one or both ends or simply is a solid elongated member such as a metallic bar or rod. The space in between the tube walls define s an annular cavity for the passage of the heat-exchanging medium. In an arrangement comprising a plurality of this type of heat transfer tubes, the solid catalyst bed is disposed only outside said tubes.
In this patent specification the terms “catalytic reactor” “heat exchange reactor” and “reactor” are used interchangeably. By “catalyst bed” is meant the volume of solid catalyst forming said bed and which is outside the heat transfer tubes and optionally, in the case of double-tubes, also inside said tubes. The terms “heat transfer tubes” and “tubes” are used interchangeably in this specification and cover any tube which is in contact with catalyst as well as a heat exchanging medium for the purpose of carrying out catalytic reactions.
A process and reactor in which a catalyst is in indirect contact with a heat exchanging medium is known from EP-A-0 271 299. This citation discloses a reactor and process that combines steam reforming and autothermal reforming. The steam reforming zone arranged in the lower region of the reactor comprise a number of tubes with catalyst disposed inside while on the upper region of the reactor an autothermal reforming catalyst is disposed outside the steam reforming tubes. EP-A-1 106 570 discloses a process for steam reforming in parallel connected tubular reformers (reactors) comprising a number of steam reforming tubes and being heated by indirect heat exchange. The catalyst is disposed in one reactor outside the steam reforming tubes and inside the steam reforming tubes in the other reactor.
In reactor configurations comprising solid catalyst particles disposed as a bed outside a plurality of heat transfer tubes, e.g. steam reforming tubes, the layout of such heat transfer tubes is of critical importance, since it would be desirable to achieve a uniform temperature distribution across the radial direction of the reactor. In other words, it would be desirable that at any given cross section along the height of the reactor, the temperature of the catalyst bed in the radial direction is kept as constant as possible.
At a given reactor length or height it is not difficult to obtain nearly uniform ratios of catalyst area (area of reactor cross section occupied by catalyst) to heat transfer area (outer surface of heat transfer tubes) and thereby uniform temperature distribution in the interior of the catalyst bed cross section, i.e. toward the centre of the reactor. This ratio can be kept constant if, for instance, the tube pitch is kept constant for the same heat transfer tube diameter. By tube pitch is meant the centre to centre distance of neighboring tubes. Even a change from for instance triangular pitch in the centre of the bed to a rectangular pitch near the periphery of the bed can be obtained without experiencing too large variations in the ratio of catalyst area to heat transfer area. However, at the periphery of the heated or cooled catalyst bed, the surrounding external reactor wall defining the periphery of the reactor does not heat nor cool the catalyst bed. In the outer periphery of said catalyst bed, that is in the regions near the external reactor wall, it can be impossible to obtain a ratio of catalyst area to heat transfer area similar to the ratio encountered toward the centre of the reactor. This is especially the case when a certain minimum distance is required between the outermost heat transfer tubes and the external reactor wall so that catalyst particles are able to surround the entire outer surface of said heat transfer tubes. If the external reactor wall is very close to or in direct contact with the outer wall of the outermost heat transfer tubes, solid catalyst particles may not be able to fit in between the wall and said tubes. Consequently dead corners or catalyst-free regions may be created. The empty space formed by these catalyst-free regions results in undesired gas channeling with concomitant undesired effects in terms of uneven flow, uneven temperature distribution in the catalyst bed as well as unconverted or less reacted process gas.